1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems include a network of base stations, base station routers, and/or other wireless access points that are used to provide wireless connectivity to access terminals in geographic areas (or cells) associated with the network. Information may be communicated between the network and the access terminals over an air interface using wireless communication links that typically include multiple channels. The channels include forward link (or downlink) channels that carry signals from the base stations to the access terminals and reverse link (or uplink) channels that carry signals from the access terminals to the base station. The channels may be defined using time slots, frequencies, scrambling codes or sequences, or any combination thereof. For example, the channels in a Code Division Multiple Access (CDMA) system are defined by modulating signals transmitted on the channels using orthogonal codes or sequences. For another example, the channels in an Orthogonal Frequency Division Multiplexing (OFDM) system are defined using a set of orthogonal frequencies known as tones or subcarriers.
Next (4th) generation wireless systems such as 802.16e WiMAX, UMTS Long Term Evolution (LTE) and cdma2000 EV-DO Revision C Ultra Mobile Broadband (UMB) are based on Orthogonal Frequency Division Multiple Access. In OFDMA, the transmitted signal consists of narrowband tones that are nearly orthogonal to each other in the frequency domain. A group of tones transmitted over the duration of one time slot (or frame) constitutes the smallest scheduling resource unit, also known as a tile, a resource block (RB), or a base node (BN). Different tones belonging to a tile may be scattered across the entire carrier frequency band used by the OFDMA system so that each tile transmission experiences diversified channels and interference on each sub-carrier. Alternatively, a tile can be formed of a contiguous set of tones so that the channel and interference experienced by the tile are more localized. Hybrid Automatic Repeat reQuest (HARQ) is employed to increase the capacity of the OFDMA system. To this end, the encoder packet transmission includes multiple HARQ interlaces repeating every certain number of frames and having a fixed maximum allowed number of sub-packet retransmissions.
Modern wireless systems typically implement coherent detection in both the forward and reverse links. This requires the transmission of pilot signals (which may also be referred to as reference signals or training signals) that can be used as a reference to estimate the amplitude and phase of the fading channel that carries the uplink/downlink signals. For example, on the reverse link, coherent detection requires that each user transmit pilot signals to its serving base station(s). The pilot signals from different mobile units (or different antennas in each mobile unit) can be multiplexed with the data and each other in time, frequency, code or a combination thereof. Typically, a set of common pilot signals are defined and divided among the various users. For example: in an OFDMA (orthogonal frequency division multiple access) system such as the one in FIG. 1, the tones are divided into data tones (open circles in FIG. 1) and common pilot tones (shaded circles in FIG. 1). Each user utilizes some of the common pilot tones, preferably those in the vicinity of its assigned data tones, to send its pilot signals. For example, user A transmits on data tones at the left side of FIG. 1 and therefore also uses the common pilots in this region. User B transmits on data tones at the right side of FIG. 1 and therefore also uses the common pilots in this region.
A growing trend in wireless systems is to equip receivers with multiple antennas. Transmissions from these users are resolved and detected by virtue of their distinct spatial characteristics across the various receive antennas (so-called spatial multiple access or virtual MIMO). This enables several users to share each time/frequency/code resource, e.g., it allows multiple users to transmit concurrently on the same frequency band and with the same code. Detecting concurrent signals is contingent on the receiver having estimates of the amplitude and phase of each of the user channels. Conventional techniques for acquiring these estimates use separate pilot transmissions from each of the users involved. The separate pilot transmissions are used to define the user channels corresponding to the multiple receive antennas. However, the pilot transmissions used to define spatial channels are not conventionally transmitted using common pilot signals because collisions between multiple users significantly reduces the ability of the receiver to detect the pilot signals of the separate users and determine the amplitude and phase of the user channels.
Wireless communication systems that implement spatially defined channels typically include dedicated pilots in addition to the common pilots. The dedicated pilot channels can be assigned to individual users so that the pilot signals transmitted on the dedicated pilot channels do not collide with signals transmitted by other users. For example: in an OFDMA (orthogonal frequency division multiple access) system such as the one in FIG. 2, the tones are divided into data tones (open circles in FIG. 2), common pilot tones (shaded circles in FIG. 2), and dedicated pilot tones (circles with shaded boundaries in FIG. 2). One range of tones on the left-hand side of FIG. 2 is assigned to Users A and C and another range of tones on the right-hand side of FIG. 2 is assigned to Users B and D. Each range of tones includes data tones, common pilot tones, and the dedicated pilot tones that can be assigned to particular users.
Allocating some of the tones to be dedicated pilot tones has a number of drawbacks. For example, the number of dedicated pilots needed to support the users in the system scales with the number of users that are performing spatial multiple access. In contrast, the number of common pilots does not scale with the number of users because all of the users can use the common pilot tones. Consequently, fewer dedicated pilot tones (relative to the potential number of common pilot tones) are typically available to each user in a system that implements dedicated pilot tones. For example, in FIG. 2, each user is allocated a single dedicated pilot tone whereas in the corresponding scenario without spatial multiple access, such as depicted in FIG. 1, each user can be allocated 2-3 common pilots. Moreover, when spatial multiple access with dedicated pilots is implemented, the common pilots are not typically used for channel estimation because of the high probability of collisions between pilot signals transmitted by different users. The common pilots may therefore represent wasted resources. Moreover, because of the reduced number of dedicated pilot channels and the unavailability of the common pilot channels, spatial multiple access systems are at a disadvantage (relative to common pilot channel systems) in terms of channel estimation because less information may be available to estimate the spatial channels.